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Well, it would be helpful to actually examine the valve but usually they either get too hot or hit something.
The overheated condition can come from inadequate heat transfer to the seat because something held it off of the seat (carbon, lack of proper clearance); or the guide is worn enough that heat can't flow from the stem to the guide; or perhaps the mixture was a little too lean in that cylinder and it ran a little (or a lot) hotter.
The impact damage scenario would involve Mr. Piston. Either we revved it too high, a bad valve spring was involved or the clearance was insufficient when the motor was built.
You shall see, when it comes out. 'Tis a real bummer.
JR
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We need not be gracious when our enemy dies. Civility is only afforded those who don't go to their grave trying to destroy us and ours. E. M. Burlingame
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